Coherent structures are structures that remain in the flow for an extended period of time and are not merely a momentary phenomena ([1]}). Considering any two-dimensional turbulent flow; there are several structures that resemble vortices and remain in the flow for a short period of time. They will have a rotary motion and a circular appearance, and if they don't endure more than a few dynamical times, they will be ephemeral, non-coherent, and classified as Eddies. If, on the other hand, an 'Eddy' tends to stay for an extended period of time and does not appear to vanish, it is no longer an Eddy but a vortex, and it is no longer transient. These structures are coherent and can persist a viscous time scale or longer, depending on the conditions. Multi-scale vorticity fluctuations are a characteristic of turbulent flows. Several methods exist for identifying vortical coherent formations. Closed-loop streamlines and pathlines, minimum pressure areas, and absolute magnitude of vorticity are examples, however since they combine rotational motion and shear, these methods do not truly identify vortex cores. Nevertheless, [2]} demonstrated that the second invariant of the rate of strain-tensor and the negative part of the second largest eigenvalue of the same tensor are reliable indicators of coherent structures in various flow situations. The second invariant in terms of streamfunction is given by the Q criterion ([3]}).
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